Shallow aquifers are the primary water source to mitigate rising hydroclimatic fluctuations in arid areas, notably in North Africa and the Arabian Peninsula. The occurrence and dynamics of these expansive water bodies remain poorly characterized due to the reliance on sporadic monitoring wells. To address this deficiency, several studies are exploring the potential of low Earth orbit sounding radars as a large-scale mapping tool that can provide unique insights into the delineation and dynamics of these aquifers. Herein, we analyze the detectability of shallow aquifers (<10 m deep) using the radiometric analysis of surface reflections from a 45-MHz orbital sounder with an 8-MHz bandwidth. We use the ray tracing method to simulate the radar return from two realistic geoelectrical and topographic models of shallow aquifers in North African Sahara desert for omnidirectional and distributed array configurations. Our results suggest that the dielectric change induced by shallow aquifers that are up to 10-m deep can increase the 45-MHz radar surface return of the desiccated desert surface by 5 dB in areas with very low surface roughness of rms height <0.35 m. These preliminary results suggest a constrained potential for a monostatic VHF reflectometry to probe large sedimentary basins, which a distributed architecture can improve.

Studies of sea level rise, ice sheet mass loss, and other glacial processes are hindered by the sparsity of observational measurements. Orbital radar sounding of the Martian ice caps has been successfully conducted, showing the promise for orbital sounding of terrestrial glaciers for improved coverage. Concepts for terrestrial orbital radar sounders, such as the Distributed Element Beamformer Radar for Ice and Subsurface Sounding (DEBRIS) mission concept, use an array of CubeSats to obtain narrow beam patterns and reduce cross-track clutter. To loosen wireless synchronization requirements, we investigate the application of orthogonal wave beamforming to DEBRIS. We compare orthogonal wave beamforming to traditional phased arrays and investigate Sinusoidal Frequency Modulated Continuous Waves (SFMCW) as an orthogonality scheme for beamforming.

Stratton-Chu radar simulations from coarsely-resolved digital elevation models (DEMs) can be accurately handled by polynomial phase approximations on the DEM facets. However, the DEM itself is only an approximation of the real terrain: a DEM is composed of planar facets and thus displays no roughness at scales below its resolution, which has important consequences on the simulated radargrams. To solve this problem, we have recomputed the linear phase approximation integral on a perturbed facet. We demonstrate the gain of fidelity over smooth-facet simulations by comparison with a real radargram. For the first time, we also apply reflectometry techniques to these simulated data, and compare it with the original output.

This paper aims to retrieve the soil moisture profile above the water table using ground penetrating radar. For the forward model, the Van Genuchten soil saturation model and the generalized refractive mixing dielectric model are used to parameterize the soil saturation profile. The finite-difference time-domain method was used to simulate the electromagnetic signal. The soil moisture profile retrieval was carried out using a look-up table. We demonstrate successful retrieval of soil moisture profile in the presence of noise.

In a recent study, a 2-D pseudospectral time-domain (PSTD) full-wave simulator was developed and demonstrated to be capable of efficiently solving large-scale low-frequency (e.g., HF) electromagnetic scattering problems, for example, on the application of radar sounding simulations of planetary clutter and subsurfaces. In this article, the 2-D PSTD simulator is applied to simulate a domain as large as 4000$\lambda $(along-track)$\times 1666.67\,\,\lambda $(cross-track)$\times 33.33\,\,\lambda $(depth) with$\lambda =15$m at an HF frequency of 20 MHz. To accomplish the goal, the simulator is further improved to efficiently model/simulate large cross-track slices of dielectric scenes by allowing nonuniform grid sampling in horizontal (lateral) and vertical directions, and the cross-track results are then stitched together along the track to form the simulated radargram. By combining the SHAllow RADar (SHARAD) viewing geometry and Mars Orbital Laser Altimeter (MOLA) digital elevation model (DEM), we simulate SHARAD returns at three different sites on Mars: one at the North Pole and two at Oxia Planum. At all three sites, the PSTD simulated radargrams are compared with measured SHARAD radargrams. Through power-level calibration and reference time adjustment, the PSTD simulated power estimates are further validated by comparing with real power observations from SHARAD with a 5-dB uncertainty and Pearson correlation coefficient of 0.3–0.4 (a$p$-value on the order of$10^{-9}$), which justifies the use of the 2-D PSTD simulator for emulating surface clutter in planetary radar sounding. This simulator is open source and can be easily modified to support radar sounding simulations in support of other planetary missions with radar sounding instruments.

Low-frequency radar sounders have the potential to generate altimetric profiles, but the feasibility of utilizing planetary radar sounding data as an alternative to laser altimetry has not been assessed using existing data to date. Therefore, we have developed, implemented, and evaluated an algorithm to process SHAllow RADar sounder (SHARAD) data on Mars (Experiment Data Records as available on the planetary data system) first into altimetry profiles and ultimately into digital terrain models (DTMs). The minimally processed data are pulse compressed, corrected for ionospheric distortion, zero-Doppler filtered, and incoherently summed. We then apply pulse re-tracking techniques adapted from terrestrial ocean altimetry to identify the surface return. From the surface return we compute the time-of-flight and hence the range from the spacecraft to the surface of the planet. The altimetry groundtracks are then co-registrated with Mars Orbiter Laser Altimeter (MOLA) to remove any biases resulting from residual ionospheric effects or timing issues. The altimetric profiles are finally used to create DTMs based on SHARAD data. While the SHARAD altimetry data have coarser inherent resolution than laser altimeters or imaging radars, we demonstrate that radar sounding data is still a viable source for satellite-based altimetry measurements. This is particularly important for future planetary missions not carrying laser altimeters but radar sounders, such as the upcoming Europa Clipper mission.

The innovations in high-performance, low-power electronics and low-cost space access are unlocking affordable distributed radar systems and new remote sensing opportunities. The Distributed Element Beamformer Radar for Ice and Subsurface sounding (DEBRIS) is a concept to implement a 2D sparse radar aperture to improve the radar's spatial resolution and sounding investigation depth through the reduction of surface clutter. Here, we introduce this system and highlight its applications, orbital configurations, and the implementation considerations to achieve state-of-the-art performance for spaceborne radar sounders.

Recent work has demonstrated a passive radio sounding approach that uses the Sun as a source for echo detection and ranging. As the Sun is a moving source with a position that is knowna priori, we evaluate this technique’s capabilities to measure the echo’s phase history, map topography, and perform synthetic aperture radar (SAR) focusing. Here, we present our approach to implementing passive SAR using a compact, temporally incoherent radio-astronomical source as a signal of opportunity. We first evaluate the passive system’s capabilities to obtain an echo from a rough surface by determining the critical signal-to-noise ratio (SNR) for reliably observing the Sun’s echo reflection with our passive instrument. We then demonstrate that our technique can detect the necessary changes in range, phase, and reflectivity of an echo from the Sun. We next present the experimental results of our passive radar testing using the Sun at Dante’s View, Death Valley, to highlight this technique’s ability to perform 2-D imaging. Finally, with synthetic data, we demonstrate that we can use time-domain backprojection to focus a planar white noise signal, perform passive SAR imaging, and improve the measurement’s SNR and azimuth resolution. The results of passive SAR focusing on white noise highlight the potential for the Sun and Jupiter’s radio emissions to perform surface and subsurface imaging for planetary and terrestrial observations.

This paper describes a 2D Pseudospectral Time-Domain (PSTD) full-wave simulator for solving large-scale half-space electromagnetic scattering problems with the application of radar sounding of planetary subsurfaces. New domain designs are developed to efficiently simulate 2D scattering of half-space media for normal and oblique incidence from arbitrary wave sources. The simulated 2D bistatic scattering radar cross width (RCW) is compared with the analytical solutions of random rough surfaces with various choices of grid sampling resolution. The PSTD solver is applied to a passive sounding problem with SAR focusing. An example of using the solver is shown for emulating three-dimensional large-scale radar sounding problems with cross-track surface and subsurface scattering. The PSTD solver is both memory-efficient and accurate for sounding applications, and particularly useful to simulate large-scale radar sounding returns and SAR focused imagery.

We report the analysis of opposite-side ambiguities in single-pass radar sounding interferometry. Nadir pointing radar sounders suffer inherent cross track ambiguities that cannot be separated with the power image alone. Interferometry can be used to interpret these ambiguities when data are collected with multichannel receivers. However, opposite-side surface clutter, volume scattering, and cross track range misalignment create unique interferometric phase and correlation signatures. Such signatures are observed experimentally in data collected by the Polarimetric Airborne Radar Ice Sounder (POLARIS) P-band multichannel radar sounder. We derive analytic expressions to predict the observations and validate them with simulation. In addition, we observe and explain a correlation dropout feature that is due to volume scattering competing with surface clutter in the null of the cross track antenna pattern. The purpose of this article is to explain opposite-side ambiguity signatures in radar sounder interferometry and provide models and techniques for scientific interpretation of interferograms.

This article discusses the implementation of a 2-D pseudospectral time-domain (PSTD) full-wave simulator for solving large-scale low-frequency (e.g., HF) electromagnetic (EM) scattering problems with the application of radar sounding of planetary subsurfaces. Compared to other computational EM algorithms, the PSTD solver is both memory-efficient and accurate for sounding applications. New domain designs are developed to efficiently simulate 2-D scattering of half-space media for normal and oblique incidence from arbitrary wave sources. As a validation of the PSTD simulator, the simulated 2-D scattering radar cross width (RCW) is compared with the analytical solutions of both point targets (dielectric cylinders) and distributed targets (random rough surfaces), for the first time, where the frequency and angular (bistatic scattering) dependence are studied with various choices of grid sampling resolution. Furthermore, the PSTD solver is applied to passive synthetic aperture radar (SAR) sounding problems (single transmitter and several receivers), for the first time, where various scenarios (e.g., cylinder, surface, and volume) are demonstrated and the targets are correctly resolved after focusing, indicating an accurate simulation of the phase history. Finally, an example of using the solver is shown for emulating 3-D large-scale radar sounding problems with cross-track surface and subsurface scattering. This is particularly useful to simulate radar sounding returns and SAR-focused imagery of large-scale subsurface structures to better support planetary missions with radar sounding instruments.

Recent work has demonstrated a passive radio sounding approach using the Sun as a source for echo detection. We expand on our passive autocorrelation-based technique by demonstrating its potential to map topography as the Sun moves throughout the entire day. Here, we show with synthetic data and our experimental results of passive radar testing with the Sun at Dante's View, Death Valley, our approach to implementing two dimensional image formation with passive radar. We also determine the critical signal to noise ratio (SNR) required to reliably observe a Sun echo with our passive instrument. Finally, we show that our autocorrelation-based technique can obtain changes in range, reflectivity, and phase, which are measurements normally acquired with traditional active radar systems. Demonstrating that our technique can acquire these changes is the first step to developing passive SAR processing using the Sun.

While radio echo sounders are powerful tools used to constrain subglacial conditions, current ice-penetrating radar systems are too resource intensive for multiyear deployment at a large scale. To address this, we present passive radio sounding as a low resource approach for observing the subsurface of ice sheets and glaciers. Although passive radar has been used for target tracking and military purposes, it has never been implemented for ice sounding. Nevertheless, recent work has proposed the passive radio sounding of Europa's icy shell using Jupiter's decametric radiation as a source for echo detection [1], [2]. Expanding on this idea, we evaluate and discuss the challenges of developing a passive radio sounder that uses the Sun for echo detection. Our prototype measures the Sun's direct and reflected path off the ocean to obtain the height of a cliff along the California coast. This serves as the first in-situ demonstration of an autocorrelation-based passive-sounder using a compact astronomical white noise source.

This paper reports the analysis of the geometric power fall-off of Fresnel zone scattering in radar sounding. Radar sounders can take advantage of strong coherent scattering from Fresnel zones at nadir which grow with sensor altitude. The strength of the signal and the actual rate of power fall-off, however, depend heavily on the surface properties. We first use the radar equation to separate geometric versus scattering mechanisms driving R2, R3, or R4fall-off. The Fresnel zone for planetary surfaces, where body curvature has an effect, is derived and its implications discussed. We show the impact of Gaussian and fractal surface roughness on the exponent of power fall-off and the transition from coherent to incoherent scattering. This is done in simulation and analytically to derive coherence loss functions. Finally, we study the effect of incoherent area fraction within the Fresnel zone. These results are intended to be used in radar link budgets, performance metrics, and scientific interpretation of sounding data.

Ice sheet contributions to sea level rise present one of the greatest challenges that our society will face in the next century. However, models predicting sea level rise due to ice melt lack critical information regarding processes at the base of ice sheets. Although radio echo sounders are powerful tools that are currently used to constrain subglacial conditions, existing ice-penetrating radar systems are too resource-intensive in terms of cost, power, and logistics for multiyear deployment at a large scale. To address this, we present passive radio sounding as a low-resource approach for observing ice sheets across a range of spatial and temporal scales. While passive radar has been used for target tracking and military purposes, it has never been used for the sounding of ice sheets. Some recent work has proposed using passive radio sounding of Europa's icy shell using Jupiter's decametric radiation. We expand on this idea by evaluating and discussing challenges and opportunities for developing a passive radio sounder using the Sun as an illuminator of opportunity for echo detection. Here, our prototype instrument sits on the side of a cliff and measures the Sun's direct and reflected path off the ocean surface. We then use an autocorrelation-based method to extract the amplitude and delay of the reflection. This serves as the first in situ demonstration of an autocorrelation-based passive sounder using a compact astronomical white noise signal.

This letter reports a method to compute the multilook interferometric phase probability density function (pdf) used in interferometric synthetic aperture radar (InSAR). The method is fast and accurate for at least 10 000 looks and the full range of interferometric correlations. This is accomplished by computing the products of large and small numbers in the underlying hypergeometric series logarithmically and by deriving a single-term recursion relation to accelerate the computation. This letter extends the usable range of inputs to the multilook interferometric phase pdf for radar systems with a large number of looks and the entire practical range of interferometric correlation.

In this paper, an airborne Ka-band (35 GHz) FMCW scatterometer and its preliminary results are presented. The paper describes the system, the calibration and the data processing needed to study the response of different targets to the scatterometer. The results obtained in the first flights include normalized radar cross-section measurements of inland water bodies and a tree height estimation algorithm.